UPS Ineffective on One Workstation

I have one workstation that will not stay powered up while connected to a UPS. Putting the UPS into test mode will kill the workstation instantly. I’m using a APC Smart 420 on a Win 98 box. The same UPS works fine on another workstation of the same make/model, and I just have the box and monitor plugged into it, so I dont think im overloading the UPS. The user also complains of a ‘hot mouse’, and I have actually felt it, and it does get kinda warm sometimes. Do you think it could be the computer power supply?

Thanks a lot,
Tom Costello